New York (CNN)The campaigns of Hillary Clinton and Bernie Sanders fought on Tuesday over which one was more credible, a volley that continued the acrimonious exchanges between the Democratic presidential candidates.

The back-and-forth started when the Sanders campaign questioned Clinton's credibility in a press release circulated by Michael Briggs, the campaign's communications director.

"Clinton's Credibility Gap," read the subject of the email, noting that on the campaign trail Tuesday, Sanders had "contrasted his record to Hillary Clinton's stands on Wall Street reform, fracking, a corrupt campaign funding system and job-killing trade deals."

The email pointed to a Washington Post article published earlier in the day that gave a "Three Pinocchio" rating -- for being "significantly misleading" -- to the former secretary of state's recent comments about guns purchased in Vermont being used for crimes in New York.

Briggs jumped on the opportunity to turn it into a question of her credibility, saying in the release that "her charge backfired."

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Clinton's campaign responded with a list of fact checks that have dinged the Vermont senator, argued that "Sanders struggled with the truth," and accused him of not running the attack-free campaign he originally said he wanted to.

"Let's be very clear. This is a character attack. This is exactly what @BernieSanders pledged to his supporters that he wouldn't do, " tweeted Nick Merrill, Clinton's traveling press secretary. "What strains credulity is pretending @BernieSanders is running the campaign they said they would. Maybe those NY lights are just too bright," he wrote.

Late into 2015, both Clinton and Sanders were running largely "positive" campaigns, with neither candidate going after the other. As contests began to stack up, however, that changed, most notably in the last two weeks of campaigning in Wisconsin and New York.

Clinton, who lives in New York and represented the state for eight years in the Senate, currently holds a double-digit lead, according to a number of polls. But Sanders has campaigned hard in the Empire State, focusing increasingly on white, working class voters in Upstate New York.

Both campaigns predict the tension will play out vividly Thursday night during the CNN/NY1 Democratic debate in Brooklyn.